46 METHODS OF STUDYING MICROORGANISMS 



this is achieved by the use of a hollow slide filled with 

 warm water. These organisms are cultivated arti- 

 ficially only with great difficulty, and the use of special 

 stains is required for the purpose of practical clinical 

 diagnosis. 



STERILIZATION. 



For a better understanding of the technic of 

 laboratory procedure, the preparation of the food- 

 stuffs or media on which bacteria thrive will be briefly 

 considered. They are prepared from meat or its 

 extracts, a substance called peptone, and salt, and 

 adjusted to a suitable reaction of weak alkalinity, 

 according to carefully worked-out formulae, which 

 are the result of long experimentation. They are 

 stored or distributed in glassware, which is of the 

 non-corrosive type. This glassware is cleaned with 

 soap and water, sand or alcohol, and rinsed with 

 distilled water. It is then sterilized by hot air. The 

 glassware and media are sterilized because bacteria 

 are ubiquitous, and apparatus and foodstuffs wholly 

 free from microorganisms are necessary in bacterio- 

 logical technic. In no other way can one be sure 

 of obtaining germs in pure culture, that is, only one 

 kind. After the medium has been put into the glass- 

 ware, steam sterilization is used ; dry heat is ineffect- 

 ual and destroys the medium. The best method of 

 sterilization is by the autoclave or pressure boiler, since 

 all organisms are killed by one atmosphere of pressure 

 to the square inch in addition to the ordinary atmos- 

 pheric pressure. Because of the delicacy of some of the 

 nutrient media it is, however, necessary to sterilize these 



